In order to improve the planeness of a rolled polyester, U.S. Pat. No. 4,140,740 proposes to set the winding tension at 100 Kp/sq.cm or more. A heat treatment method where the air amount intervening in the roll is prescribed is also known as described in JP-A-64-5825 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
On the other hand, although the polyester support has good mechanical strength and is suitable for the thinning, it suffers from such a defect as that the curling habit is not released.
In order to overcome such a defect, U.S. Pat. No. 4,141,735 describes a method where a polyester-based polymer is heat treated at a temperature (hereinafter referred to as "BTA" meaning "below Tg annealing") lower than the glass transition temperature (hereinafter referred to as "Tg") to become difficult to have a curling habit.
When it is subjected to heat treatment in a roll form, the equipment required for the heat treatment is only a thermostatic bath and such a heat treatment method is advantageous in view of industrial point, however, the planeness is conspicuously lowered due to the deformation of film accompanying shrinkage and expansion generated during heat treatment. For example, wavy unevenness may be caused at a cycle of several tens cm in the widthwise direction or recessions in a diameter of about several cm may be generated periodically.
Accordingly, the polyester support cannot be used as a photographic support which is required to have higher planeness.